Sixty strains of root nodule bacteria were screened for siderophore product
ion in low-iron broth, among them 40 strains from the Australian Inoculants
Research and Control Service (AIRCS) which are the current commercial inoc
ulants used in the pulse and legume pasture industries in Australia. Eleven
new siderophore-producing strains were recognised including Sinorhizobium
meliloti (WSM826. WSM352, SU47), Rhizobium leguminosarum boivar viviae (WU1
63, MNF3841, SU391), Rhizobium leguminosarum boivar trifoli (CB782, CC2483g
, CC383b) and Rhizobium tropici (WSM1385, CB3060). Siderophores were identi
fied by chemical characterisation for catecholate or hydroxamate, spectral
studies, isoelectrofocusing and siderophore-mediated iron-uptake studies. T
he S. meliloti strains all produced dihydroxyamate siderophores. Other side
rophore-producing rhizobia, with the exception of R. tropici CB306c, excret
ed trihydroxamate-type siderophores. No bradyrhizobia were Chromazurol S-po
sitive. Fe-59 uptake studies revealed that all strains transported iron com
plexed to citrate. The sinorhizobia tool; up 5-10-fold more iron from dihyd
roxamate than trihydroxamate siderophores. Conversely, other rhizobia and t
he slow-growing bradyrhizobia transported iron complexed to trihydroxamates
at rates 2-5 fold those of dihydroxamate siderophores. Rhizobactin 1021 wa
s excreted by S. meliloti strains 1021, Rm2011 and SU47 and vicibactin by s
even strains of R. leguminosarum (bv. viciae and bv. trifolii). (C) 2000 El
sevier Science Ltd. All rights reserved.